A superconducting accelerating cavity is to accelerate charged particles which travel through the inside of the superconducting accelerating cavity. A beam pipe provided at the end of the superconducting accelerating cavity is equipped with a harmonic (HOM) coupler for removing harmonics which disturb beam acceleration (i.e., for extracting the harmonics, which are induced in the superconducting accelerating cavity, out of the superconducting accelerating cavity) and an input coupler for inputting microwaves into a cavity body. The input coupler is flange-coupled to an input port attached to the beam pipe (see, for example, Patent Literature 1). The higher order mode coupler is flange-coupled to a pickup port having a pickup antenna for extracting harmonics to the outside, the pickup antenna being attached to a lateral part of an outer conductor.
Conventionally, the pickup port is configured as shown in FIG. 4 in one example. The pickup port 71 is composed of a generally cylinder-shaped port body 73 and a flange portion 75 attached to an outer peripheral side of one end of a port body 73 by welding such as electron beam welding. The other end of the port body 73 is welded by, for example, electron beam welding to a port portion 79 which is formed so as to go through a lateral face of an outer conductor 77.
The flange portion 75 is firmly attached by using bolts to a flange of a pickup antenna side through a sealing member. Accordingly, the flange portion 75 has a through hole 81 formed for the bolts. The input port is also made to have the same configuration as the pickup port.
The accelerating cavity body, the beam pipe, the higher order mode coupler, and the port body 73 are formed of a superconducting material such as niobium materials having high purity (e.g., 99.85% or more). The flange portion 75 is formed of, for example, a niobium titanium alloy having a titanium content of 45 to 55%.
The sealing member is made use of, for example, a material such as metal O rings which require high planar pressure and have high sealability. The flange portion 75 needs predetermined hardness in order to compress the sealing member, and so the sealing member is formed of a niobium titanium alloy.
When the inside of the superconducting accelerating cavity is purified by electric polishing after assembly, the niobium titanium alloy may possibly corrode by polishing liquid. Accordingly, the flange portion 75 is attached to an outer peripheral side of the port body 73 so as not to come into contact with the polishing liquid.